At present most detectors used are photometers and refractometers, and more rarely, for very specific applications, electrochemical, conductimetric detectors or others.
Photometers offer the advantages of a high sensitivity and a great stability. They can be used in all cases where the eluates to be detected absorb light in the range of wavelengths between about 190 and 700 nm. But these photometers have major drawbacks: they are not universal, and in the same analysis, it is possible to encounter not only eluates that absorb light at different wavelengths or which practically do not absorb light, which particularly in preparative chromatography can have the drawback of allowing impurities to pass unperceived.
What is more, in this case of preparative chromatography, the photometer is quickly saturated at about an optical density on the order of 2.
By dividing the length of the optical path in the cuvette by 10, this drawback is reduced, but the device loses even more sensitivity.
In regard to refractometers, they have the essential advantage of being practically universal. Most of the devices now marketed are with light beam deviation by double prismatic cuvette or container with circulation. In these detectors, a light source projects a beam onto a double photodetector after having successively gone through a diaphragm, optional concentration lenses, a turning glass plate with parallel faces to adjust the optical zero of the device, i.e., to balance the light intensity illuminating the two photodetectors, a double prismatic cuvette, one for a reference liquid, the other for the phase to be analyzed.
When the index of refraction of this latter varies, the prismatic section of the two successive cuvettes is such that the beam deviates from one photodetector to the next, according to the sign of the difference of the indices between the two cuvettes. Now, in some cases, particularly in preparative chromatography, where it is possible to encounter high concentrations in one of the two cuvettes, or if an elution gradient is created, the variations of the indices can be such that the beam deviates up to the saturation of the instrument, i.e., the deviated beam illuminates only one of the two cells. Thus, the chromatogram is clipped and several peaks with a common base can be undistinguished. Just as for photometers, it is possible to reduce the drawbacks by reducing the deviations, but at the price, here, too, of loss of sensitivity. The saturation is then avoided, but small peaks, i.e., impurities, are no longer distinguished when using preparative chromatography.
Regardless of the efforts made, the problems created by a gradient are encountered.
A monochromatic source refractometer system has also been proposed whose beam is divided to go through two cuvettes in parallel, for a reference liquid and the phase to be analyzed, then the two beams are gathered to illuminate a photodetector. Interferences are produced because of the variation of the optical path on the measurement side as a function of the variation of the index. It can be considered that the sine wave followed by the intensity is linear in the vicinity of the zero index difference; this gives an acceptable sensitivity but does not solve the problems of saturation and use of gradient, and as in the previous cases, the saturation can be avoided to the detriment of sensitivity.